On the existing aircrafts, upon a track or heading to be modified, being captured in a flight control unit of the FCU (“Flight Control Unit”) type, a usual displaying system generally visualizes on a navigation screen of the ND (“Navigation Screen”) type:                the heading (or the track) directive; as well as        the present heading and the continuation of the current track.        
Thus, when the heading or the track directive is modified, the aircraft is turning, but the ND navigation screen displays a straight line segment indicating the current track. This straight line segment does not show the trajectory according to which the aircraft will fly (as the latter will follow a curved trajectory until the new directive, then a rectilinear trajectory), but it only illustrates an indication of the instantaneous track at a given instant. Consequently, the usual displaying devices do not allow a visualization of the lateral trajectory the aircraft will actually follow in such a situation.
Moreover, upon an approach, this straight line segment remains displayed when the crew arm an approaching mode for aligning the aircraft on the axis of a landing runway (for example thru pushing on a button of the <<APPR>> or <<LOC>> type on the flight control unit FCU). In such a case, the navigation screen displays a rectilinear trajectory (extrapolation of the current track), while the aircraft will follow this trajectory only until the capture point for the LOC beam, where the guidance objective will change. The usual displaying devices therefore do not allow a visualization of the trajectory that the aircraft will follow, upon the capture of the axis of a runway upon landing.
In addition, a short term path predictor is known (referred to as <<path predictor>>) that can be displayed. However, this predictor:                is limited to a prediction for a given flight time, for example 60 seconds; and        is represented as a circle arc, the curvature radius of which is calculated from the current angle of roll of the aircraft and from the current ground speed.        
This predictor therefore only provides an indication on the curvature radius being able to be followed from the current angle of roll of the aircraft. It does not translate, in any case, the reality of the trajectory of an aircraft even at short term. For example, upon approaching the directive of heading, the aircraft will decrease its roll, but the predictor does not anticipate such a change of trajectory.
Upon a modification of the lateral directive (HEADING or RUNWAY mode) on the flight control unit FCU, the crew could therefore actually visualize the direction the aircraft will head to at short term, the point where the aircraft is turning, but have no accurate and immediate information available on the trajectory the aircraft will fly, whatever the modes of the automatic piloting system (prediction of the trajectory as early as the modification of the directive, detection of the end of a roll, real impact of turning and end turning, etc.).
In addition, upon such a modification of the lateral directive when the speed of the aircraft is varying, the pilot has no information available on the impact of the evolution of the speed on the lateral trajectory.
On the other hand, the pilot does not have any information, at the level of the displaying device, relating to the requested rotation direction of a turn. For example, in the case where the aircraft is turning to the left (with a negative current angle of roll—anticlockwise rotation), and the pilot captures a directive higher than 180° of the current heading performing a clockwise rotation of the selector, the predictor indicates an immediate turn in the anticlockwise direction before reversing upon the change of turning direction of the aircraft, according to the directive entered by the pilot.
In addition, when a change of mode is imminent, the pilot does not have available, in all guiding modes, information relating to the moment where this change of mode will occur and to its impact on the flight of the aircraft. For example, if all conditions for the engagement of a mode of capture of a runway axis are about to be gathered, the pilot does not have available information for checking whether the mode of capture of axis will actually engage or not, when the aircraft will start to initiate its turn for getting aligned on the runway, or even, in the case of a high speed or of a late arming of the approaching mode, whether the aircraft will overcome the approach axis.
Now, more and more data are displayed on navigation screens, such as the flight plane with respect to the ground, a meteorological map, the relief, the landing runways, the areas of air control, etc. The crew is therefore bound to take decisions relating to such data as referenced with respect to the ground, but, at the same time, they do not have available information on the predictable trajectory of the aircraft referenced with respect to the ground in all guiding modes available with the automatic piloting system, and this, already upon the change of mode and/or directive captured on the flight control unit. They cannot therefore check that the directives and the selection of mode are in accordance with their intentions, nor even optimize their track.
The present invention relates to a method for aiding the flight management of an aircraft, in particular a transport airplane, being able to be guided according to one of a plurality of different guiding modes, allowing the above mentioned drawbacks to be overcome.